San Diego Zoo scientists have discovered that elephants talk to each other in a secret language. In soon-to-be-published findings, they learned that the giant mammals can communicate using a frequency too low to be heard by humans or their natural predators. So, maybe “Horton Hears a Who!” isn’t fiction, after all…

While trumpeting is a familiar sound, it has long been known that elephants communicate with low growling or “rumbling” noises. People can only hear the top third of the register; two-thirds occurs at too low a frequency for human ears.

New research shows that elephants manipulate this low-frequency rumbling to impart different messages, said Matt Anderson, associate director of behavioral biology at the zoo’s Institute for Conservation Research.

For instance, females broadcast to distant males that they are ready to mate. The low frequency sound, made possible because of the gigantic size of elephants’ lungs and vocal cords, can be heard by elephants several miles away.

For instance, 10 days to two weeks before giving birth, a mother-to-be starts broadcasting the impending arrival of her calf. By “eavesdropping” electronically, keepers also can be better prepared for that birth.

“We believe there is a lot of manipulation at these very low frequencies,” says Anderson. His research is ongoing in hopes of identifying an entire vocabulary of expressions, maybe an elephant alphabet.